Pension Plans and Other Postretirement Benefits - Schedule of Projected Benefit Obligation, Accumulated Benefit Obligation, and Fair Value of Plan Assets (Details) - USD ($) $ in Millions	Sep. 30, 2019	Sep. 30, 2018
Defined Benefit Plan Disclosure [Line Items]
Projected benefit obligation	$ 751.4	$ 664.6
Accumulated benefit obligation	712.9	636.0
Fair value of plan assets	521.8	499.2
Spire Missouri
Defined Benefit Plan Disclosure [Line Items]
Projected benefit obligation	538.4	473.3
Accumulated benefit obligation	500.2	446.5
Fair value of plan assets	379.2	349.1
Spire Alabama
Defined Benefit Plan Disclosure [Line Items]
Projected benefit obligation	152.5	136.3
Accumulated benefit obligation	152.3	134.5
Fair value of plan assets	$ 93.3	$ 101.3

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Amount of projected benefit obligation for defined benefit pension plan with projected benefit obligation in excess of plan assets.

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Amount of accumulated benefit obligation for defined benefit plan with accumulated benefit obligation in excess of plan assets.

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-Publisher FASB
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Amount of plan asset for defined benefit plan with accumulated benefit obligation in excess of plan assets.

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